The origin of carbonates in termite mounds of the Lubumbashi area, D.R. Congo

The origin of carbonate accumulations in termite mounds is a controversial issue. This study is an attempt to elucidate the processes of carbonate precipitation in Macrotermes mounds built on Ferralsols in Upper Katanga, D.R. Congo, whereby a differentiation between pedogenic and inherited carbonates is considered. Carbonate features were investigated for a 9 m deep termite-mound profile, and for an 18 m wide cross-section through a termite mound and the adjacent soil, using field and laboratory techniques. Field evidence for a pedogenic origin includes morphological type (soft powdery materials, nodules, and coatings on ped surfaces) and distribution patterns of the carbonates. Thin-section studies reveal that the carbonates occur predominantly as impregnative orthic nodules and less commonly as coatings, both clearly pedogenic; calcareous pellets are interpreted as locally reworked pedogenic carbonates. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDS) and stable isotope (δ¹³C) analyses show that all isolated carbonate features consist of high-Mg calcite (4.9-12.3 mol% MgCO₃) with δ¹³C signatures ranging from − 13.2‰ to − 11.5‰. Weddellite (CaC₂O₄. 2H₂O) is identified in a thin-section and by XRD analysis, and appears to be locally transformed into calcite. The stable isotope composition of carbon suggests that calcite precipitated in equilibrium with soil CO₂ generated during decomposition of soil organic matter, and locally most likely during oxidation of oxalate. This study proves that carbonates which accumulated in Macrotermes mounds are pedogenic precipitates, whose deposition is partly related to microbial decay of organic matter, subsequently redistributed to some extent by abiotic dissolution-reprecipitation and termite activity.     

荒漠草原土壤次生碳酸盐形成和周转过程中固存CO2的研究

次生碳酸盐是干旱、半干旱地区土壤碳库的一个重要组成部分,较有机碳库大2倍~5倍,其形成与周转对该地区土壤碳平衡具有十分重要的意义。本文以内蒙古四子王旗短花针茅荒漠草原为研究对象,拟通过分析测定土壤碳酸盐(SC)含量及其δ13C值、土壤CO2的δ13C值和母质风化物的δ13C值等,探讨土壤次生碳酸盐形成和重结晶过程中对土壤CO2的固定,并计算其固定量。结果表明:次生碳酸盐的δ13C值主要由土壤CO2的碳同位素组成决定,可用土壤CO2的δ13C值计算次生碳酸盐的δ13C值。对该研究区SC进行区分,发现在40~50、50~60、60~80和80~100 cm土层,次生碳酸盐所占的比例分别为83.6%、65.4%、41.7%和17.4%。应用δ13C值和化学平衡原理估算该地区次生碳酸盐形成和重结晶过程中固定的CO2量,得出40~50、50~60、60~80和80~100 cm土层每千克土壤中分别固定了38.3、39.2、25.1和9.2 g的土壤CO2。13     

Probing carbon flux patterns through soil microbial metabolic networks using parallel position-specific tracer labeling

In order to study controls on metabolic processes in soils, we determined the dynamics of ¹³CO₂ production from two position-specific ¹³C-labeled pyruvate isotopologues in the presence and absence of glucose, succinate, pine, and legume leaf litter, and under anaerobic conditions. We also compared ¹³CO₂ production in soils along a semiarid substrate age gradient in Arizona. We observed that the C from the carboxyl group (C₁) of pyruvate was lost as CO₂ much faster than its other C atoms (C_2,3). Addition of glucose, pine and legume leaf litter reduced the ratio between ¹³CO₂ production from 1-¹³C pyruvate and 2,3-¹³C pyruvate (C₁/C_2,3 ratio), whereas anaerobic conditions increased this ratio. Young volcanic soils exhibited a lower C₁/C_2,3 ratio than older volcanic soils. We interpret a low C₁/C_2,3 ratio as an indication of increased Krebs cycle activity in response to carbon inputs, while the higher ratio implies a reduced Krebs cycle activity in response to anaerobic conditions. Succinate, a gluconeogenic substrate, reduced ¹³CO₂ production from pyruvate to near zero, likely reflecting increased carbohydrate biosynthesis from Krebs cycle intermediates. The difference in ¹³CO₂ production rate from pyruvate isotopologues disappeared 4-5 days after pyruvate addition, indicating that C positions were scrambled by ongoing soil microbial transformations. This work demonstrates that metabolic tracers such as pyruvate can be used to determine qualitative aspects of C flux patterns through metabolic pathways of soil microbial communities. Understanding the controls over metabolic processes in soil may improve our understanding of soil C cycling processes.     

The influence of precipitation pulses on soil respiration - Assessing the “Birch effect” by stable carbon isotopes

Sudden pulse-like events of rapidly increasing CO₂-efflux occur in soils under seasonally dry climates in response to rewetting after drought. These occurrences, termed “Birch effect”, can have a marked influence on the ecosystem carbon balance. Current hypotheses indicate that the “Birch” pulse is caused by rapidly increased respiration and mineralization rates in response to changing moisture conditions but the underlying mechanisms are still unclear. Here, we present data from an experimental field study using straight-forward stable isotope methodology to gather new insights into the processes induced by rewetting of dried soils and evaluate current hypotheses for the “Birch“-CO₂-pulse. Two irrigation experiments were conducted on bare soil, root-free soil and intact vegetation during May and August 2005 in a semi-arid Mediterranean holm oak forest in southern Portugal. We continuously monitored CO₂-fluxes along with their isotopic compositions before, during and after the irrigation. δ¹³C signatures of the first CO₂-efflux burst, occurring immediately after rewetting, fit the hypothesis that the “Birch” pulse is caused by the rapid mineralization of either dead microbial biomass or osmoregulatory substances released by soil microorganisms in response to hypo-osmotic stress in order to avoid cell lyses. The response of soil CO₂-efflux to rewetting was smaller under mild (May) than under severe drought (August) and isotopic compositions indicated a larger contribution of anaplerotic carbon uptake with increasing soil desiccation. Both length and severity of drought periods probably play a key role for the microbial response to the rewetting of soils and thus for ecosystem carbon sequestration.     

基于文献计量分析土壤无机碳研究现状与趋势

土壤无机碳在固碳、维持土壤质量、促进有机碳稳定性与肥力和生态系统服务功能等方面有重要作用。以Web of Science核心合集数据库和CNKI数据库中472篇土壤无机碳研究的文献为数据源,通过VOSviewer、CiteSpace和HistCite分别从发文量、被引频次、合作强度及关键词等角度对发文国家、机构、作者、研究热点及其变化趋势进行计量分析与可视化展示。结果表明：2003 ~ 2023 a期间土壤无机碳研究的发文数量不断增长,主要从土壤学与环境学向其他学科延伸;科研机构以中国科学院为首,而发文作者形成了以Kuzyakov Y为核心的作者群,总体而言我国的发文量和国际影响力最大。当前的研究热点集中在土壤无机碳储量及其影响因素,土壤无机碳在碳循环中的作用及其固碳潜力,土壤无机碳的生态系统服务功能等方面。然而,当前对碳酸盐再沉淀过程中有机碳向无机碳的转移及固碳作用研究不足,尤其是植物光合同化碳向土壤次生碳酸盐累积转化及对氮肥的响应机制尚不清楚。今后应通过实地研究收集无机碳数据并结合机器学习建模等方法,提高估算全球碳库及碳周转速率的准确性;深化土壤有机碳-二氧化碳-土壤无机碳的转化机理,进一步明晰无机碳对稳定土壤碳库和维持土壤健康等生态服务方面的作用。     

稳定性同位素技术在土壤重要有机组分循环转化研究中的应用

土壤有机质包含多种不同结构的有机物质,如碳水化合物(中性糖、氨基糖)、蛋白质(氨基酸)、木质素等,这些组分在土壤中的保留时间从几天到几百万年,在有机质循环过程中的积累和转化动态各有特征,其作用和贡献也有所不同.由于有机质各组分本身在土壤中稳定存在,只有利用同位素示踪技术才能定量研究有机质及其组分的循环动态.本文概述了利用稳定同位素示踪技术研究有机质中的一些重要组分的来源、可利用性和转化动态及其生物标识作用的进展情况,从而深入了解土壤有机质循环转化动态及调控机制.     

土壤水稳定同位素研究进展

本文从影响土壤水稳定同位素变化的因素、时空变化规律以及\"土壤-植物-大气\"界面水分转化和循环过程等方面综述了国内外土壤水同位素研究的主要成果,认为同位素方法在研究土壤水运移、降水入渗及土壤蒸发问题上优势比较明显,并且有助于从宏观和微观上阐明土壤水的特征及其运动规律;指出了当前土壤水分同位素研究存在的问题和今后研究的重点,并对同位素技术与方法在土壤水中应用前景进行了展望。     

Spatial variation of soil organic carbon concentrations and stable isotopic composition in 1-ha plots of forest and pasture in Costa Rica: implications for the natural abundance technique

The goal of this study was to examine spatial variation of soil organic C and its stable isotopic composition (δ¹³C) in 1-ha plots of mature rain forest and a cattle pasture dominated by C4 grasses in Costa Rica. Soil samples were taken from 80 mapped locations per plot and analyzed for organic C and δ¹³C. The range of values for soil C concentrations was similar between forest and pasture, although the mean values were higher in the forest. δ¹³C was narrowly constrained in the forest (from −27.96 to −26.09‰) but varied from −15.09 to −28.59‰ in the pasture. Variograms revealed spatial autocorrelation of soil C and δ¹³C in the pasture and organic C concentration in the forest soil. The large range and spatial variability of δ¹³C in the pasture site may be due to varying contributions of C3 and C4 vegetation to the soil C pool, which may limit the usefulness of the natural abundance technique as a precise tracer of soil C dynamics in this pasture.     

Nano-scale secondary ion mass spectrometry — A new analytical tool in biogeochemistry and soil ecology: A review article

Soils are structurally heterogeneous across a wide range of spatio-temporal scales. Consequently, external environmental conditions do not have a uniform effect throughout the soil, resulting in a large diversity of micro-habitats. It has been suggested that soil function can be studied without explicit consideration of such fine detail, but recent research has indicated that the micro-scale distribution of organisms may be of importance for a mechanistic understanding of many soil functions. Current techniques still lack the adequate sensitivity and resolution for data collection at the micro-scale, and the question ‘How important are various soil processes acting at different scales for ecological function?’ is therefore challenging to answer. The nano-scale secondary ion mass spectrometer (NanoSIMS) represents the latest generation of ion microprobes, which link high-resolution microscopy with isotopic analysis. The main advantage of NanoSIMS over other secondary ion mass spectrometers is its ability to operate at high mass resolution, whilst maintaining both excellent signal transmission and spatial resolution (down to 50 nm). NanoSIMS has been used previously in studies focussing on presolar materials from meteorites, in material science, biology, geology and mineralogy. Recently, the potential of NanoSIMS as a new tool in the study of biophysical interfaces in soils has been demonstrated. This paper describes the principles of NanoSIMS and discusses the potential of this tool to contribute to the field of biogeochemistry and soil ecology. Practical considerations (sample size and preparation, simultaneous collection of isotopes, mass resolution, isobaric interference and quantification of the isotopes of interest) are discussed. Adequate sample preparation, avoiding bias due to artefacts, and identification of regions-of-interest will be critical concerns if NanoSIMS is used as a new tool in biogeochemistry and soil ecology. Finally, we review the areas of research most likely to benefit from the high spatial and high mass resolution attainable with this new approach.     

Purification and isotopic signatures (<Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C, <Emphasis Type="Italic">δ</Emphasis><Superscript>15</Superscript>N, Δ<Superscript>14</Superscript>C) of soil extracellular DNA

The aim of this work was to obtain pure extracellular DNA molecules so as to estimate their longevity in soil by an isotope-based approach. Extracellular DNA molecules were extracted from all horizons of a forest soil and purified by the procedure of Davis (Purification and precipitation of genomic DNA with phenol–chloroform and ethanol. In: Davis LG, Dibner MD, Battey JF (eds) Basic methods in molecular biology. Appleton & Lange, Norwalk, 16–22, 1986) without (DNA1) or with (DNA2) a successive treatment with binding resins followed by elution. The two differently purified DNA samples were compared for their A₂₆₀/A₂₈₀ ratio, polymerase chain reaction (PCR) amplification and natural abundance of stable (¹³C and ¹⁵N) and radioactive (¹⁴C) isotopes. The purity index and the PCR amplification did not differentiate the efficiency of the two purification procedures. The isotopic signature of DNA was more sensitive and was strongly affected by the purification procedures. The isotopic measurements showed that the major contaminant of extracellular DNA1 was the soil organic matter (SOM), even if it is not possible to exclude that the similar δ ¹³C, δ ¹⁵N and Δ¹⁴C values of DNA and SOM could be due to the use of SOM-deriving C and N atoms for the microbial synthesis of DNA. For extracellular DNA2, extremely low values of Δ¹⁴C were obtained, and this was ascribed to the presence of fossil fuel-derived substances used during the purification, although in amounts not revealed by gas chromatography-mass spectrometry analysis. The fact that it is not possible to obtain contaminant-free DNA molecules and the potential use of soil native organic compounds during the microbial synthesis of DNA make it not achievable to estimate the age of soil extracellular DNA by radiocarbon dating.     

Experiments on the origin of C in the calcium carbonate granules produced by the earthworm Lumbricus terrestris

Granules of calcium carbonate up to 2.5 mm in size have been found in archaeological soils and sediments for many years. Recent work has shown conclusively that these granules are derived from the calciferous glands of earthworms and most commonly (in Northern European soils) from Lumbricus terrestris. Valuable pedological and archaeological information could be obtained from a radiocarbon date if the ¹⁴C in the granule carbonate reflects the true age of formation. Preliminary information on the possible sources of carbon in the granule calcite was obtained by a series of simple experiments growing worms in artificial substrates made up of materials with marked differences in δ¹³C. The results showed that carbon in the granule calcite comes from the dietary inputs, but may be coming partly from atmospheric CO₂ and also partly from old soil carbon. Little appears to come from the soil calcium carbonate. Most of the results imply that a fractionation of around +12‰ is occurring between the carbon ingested by the worm and the carbon output in the granules. This is comparable to (but slightly higher than) the widely quoted non-biological value (ca. +10‰) of total kinetic fractionation between CO₂-CaCO₃.     

Partitioning soil carbon responses to warming: Model-derived guidance for data interpretation

Parallel incubation at different temperatures combined with ¹³CO₂ efflux has been used to distinguish the temperature sensitivity of labile soil carbon (young soil carbon derived from newly-introduced vegetation) from that of resistant soil carbon (old, native vegetation-derived soil carbon). But we believe that this approach to assessing relative temperature sensitivities is confounded by differential rates of depletion of labile and resistant soil carbon at different temperatures. Here we employ a simple decomposition model to demonstrate potential pitfalls in interpreting ¹³CO₂ efflux data that inevitably, and potentially erroneously, lead to the conclusion that decomposition of resistant soil carbon pools is more temperature sensitive than labile pools. We conclude by offering a new approach for interpreting these data that eliminates this potential bias.     

The classification, distribution, and extent of soils with a xeric moisture regime in the United States

Soils with cool, moist winters and relatively warm, dry summers, a Mediterranean climate, are recognized as having a xeric moisture regime in Soil Taxonomy. These soils are classified mostly in taxa that use the formative element “xer” in the name. Soil series with either a xeric moisture regime or an aridic regime bordering on a xeric moisture regime make up more than 48,640,000 hectares in the western part of the United States. They are classified in the orders of Mollisols (20,080,000 ha), Aridisols (11,200,000 ha), Alfisols (5,320,000 ha), Inceptisols (4,800,000 ha), Entisols (4,400,000 ha), Vertisols (1,520,000 ha), Andisols (960,000 ha), and Ultisols (960,000 ha).     

Modeling soil metabolic processes using isotopologue pairs of position-specific C-labeled glucose and pyruvate

Most organic carbon (C) in soils eventually turns into CO₂ after passing through microbial metabolic pathways, while providing cells with energy and biosynthetic precursors. Therefore, detailed insight into these metabolic processes may help elucidate mechanisms of soil C cycling processes. Here, we describe a modeling approach to quantify the C flux through metabolic pathways by adding 1-¹³C and 2,3-¹³C pyruvate and 1-¹³C and U-¹³C glucose as metabolic tracers to intact soil microbial communities. The model calculates, assuming steady-state conditions and glucose as the only substrate, the reaction rates through glycolysis, Krebs cycle, pentose phosphate pathway, anaplerotic activity through pyruvate carboxylase, and various biosynthesis reactions. The model assumes a known and constant microbial proportional precursor demand, estimated from literature data. The model is parameterized with experimentally determined ratios of ¹³CO₂ production from pyruvate and glucose isotopologue pairs. Model sensitivity analysis shows that metabolic flux patterns are especially responsive to changes in experimentally determined ¹³CO₂ ratios from pyruvate and glucose. Calculated fluxes are far less sensitive to assumptions concerning microbial chemical and community composition. The calculated metabolic flux pattern for a young volcanic soil indicates significant pentose phosphate pathway activity in excess of pentose precursor demand and significant anaplerotic activity. These C flux patterns can be used to calculate C use efficiency, energy production and consumption for growth and maintenance purposes, substrate consumption, nitrogen demand, oxygen consumption, and microbial C isotope composition. The metabolic labeling and modeling methods may improve our ability to study the biochemistry and ecophysiology of intact and undisturbed soil microbial communities.     

Erosion rates and nutrient losses affected by composted cattle manure application in vineyard soils of NE Spain

Land preparation for mechanisation in vineyards of the Anoia–Alt Penedès region, NE Spain, has required major soil movements, which has enormous environmental implications not only due to changes in the landscape morphology but also due to soil degradation. The resulting cultivated soils are very poor in organic matter and highly susceptible to erosion, which reduces the possibilities of water intake as most of the rain is lost as runoff. In order to improve soil conditions, the application of organic wastes has been generalised in the area, not only before plantation but also every 3–4 years at rates of 30–50 Mg ha^− 1 mixed in the upper 30 cm.These organic materials are important sources of nutrients (N and P) and other elements, which could reduce further fertilisation cost. However, due to the high susceptibility to sealing of these soils, erosion rates are relatively high, so a higher nutrient concentration on the soil surface increases non-point pollution sources due to runoff.The aim of this study is to analyse the influence of applied composted cattle manure on infiltration, runoff and soil losses and on nutrients transported by runoff in vineyards of the Alt Penedès–Anoia region, NE Spain. In the two plots selected for the analysis, composted cattle manure had been applied in alternate rows 1 year previous to the study. In each plot soil surface samples (0–25 cm) were taken and compared to those of plots without manure application. The study was carried out at laboratory scale using simulated rainfall. Infiltration rates were calculated from the difference between rainfall intensity and runoff rates, and the sediment and total nitrogen and phosphorus were measured for each simulation. In addition, the influence of compost was investigated in the field under natural rainfall conditions by analysing the nutrient concentration in runoff samples collected in the field (in the same plots) after seven rainfall events, which amount different total precipitation and had different erosive character.Compost application increases infiltration rates by up to 26% and also increases the time when runoff starts. Sediment concentration in runoff was lower in treated (13.4 on average mg L^− 1) than in untreated soils (ranging from 16.8 to 23.4 mg L^− 1). However, the higher nutrient concentration in soils produces a higher mobilisation of N (7–17 mg L^− 1 in untreated soils and 20–26 mg L^− 1 in treated soils) and P (6–7 mg L^− 1 in untreated soils and 13–19 mg L^− 1 in treated soils). A major part of the P mobilised was attached to soil particles (about 90% on average) and only 10% was dissolved. Under natural conditions, higher nutrient concentrations were always recorded in treated vs. untreated soils in both plots, and the total amount of N and P mobilised by runoff was higher in treated soils, although without significant differences. Nutrient concentrations in runoff depend on rainfall erosivity but the average value in treated soils was twice that in untreated soils for both plots.